This invention relates to a device using a television camera tube, and more particularly to a television camera tube device suitable for controlling the quantity of electron beam current depending on the luminous intensity of an object.
In a vidicon type television camera tube, a charge pattern corresponding to the luminous intensity of a moving object is produced on a photoconductive layer target, and an electron beam generated from an electron gun is directed to scan the photoconductive layer target, thereby to cause successive discharge of the charge pattern. A charge current corresponding the above discharge is taken out of the television camera tube as a signal. All of the charges produced on the target by the object in each beam scanning operation are not completely discharged after the beam scanning. As a result, an unfavorable signal corresponding to the residual charges appears as a beam discharge lag in the next and succeeding scanning thereby degrading the picture quality of the moving object. Especially, in a television camera tube using a blocking type photoconductive layer target, the beam discharge lag is caused principally by a capacitive signal lag having a time constant determined by the product of the electrostatic capacitance of the photoconductive layer and the beam resistance of the scanning electron beam. The beam resistance is equivalent to the velocity distribution of the electron group forming the electron beam. Therefore, it is essentially required to constrict the velocity distribution of the electron group forming the electron beam in order to minimize the beam discharge lag.
The electron group emitted from the cathode of the electron gun has a velocity distribution in the form of the Maxwell's distribution. It is known that, in the course of formation of a fine electron beam, the current density of the beam increases, and the velocity distribution of the beam is broadened by energy relaxation due to the Coulomb's force acting between the electrons. This phenomenon is called the Boersh effect, and the broadening rate of the velocity distribution of the beam is generally proportional to J(z).sup.1/3, when J(z) is the beam current density on the tube axis.
Therefore, in a television camera tube intended to minimize the beam discharge lag, it is necessary to prevent an undesirable increase in the beam current density as much a possible. For this purpose, a diode type electron gun has been proposed in which a first grid opposing the cathode is operated at a voltage positive relative to the cathode to cause emission of electrons from the cathode in parallel to the tube axis, thereby generating a laminar flow electron beam which does not form a crossover having a high current density. (Refer to, for example, U.S. Pat. No. 3,894,261.) However, in such a diode type electron gun generating a laminar flow electron beam, the beam current quantity is proportional to the emission current density of the cathode, and, therefore, the current density of the cathode becomes extremely high for obtaining a large beam current. Thus, it has been difficult to permit the operation of automatic beam optimizer (ABO) in which the dynamic range of the beam current quantity is widened so as to control the beam quantity according to the luminous intensity of an object.